1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of controlling a chemical mechanical polishing for forming a conductive plug or planarizing an insulation layer.
2. Description of the Related Art
Chemical Mechanical Polishing (CMP) is a process that is useful to planarize an insulation layer or to form a conductive plug. It is necessary that the CMP be completed at a proper time in order to polish a target material down to a predetermined thickness.
Methods for controlling an end of a CMP process may be classified into two types. The first type is a time controlled method, in which the polishing is performed for a predetermined time that has been previously set. The second type is an end point detection method, which uses an end point detector. The end point detection method has an advantage in that it is able to detect the end point of the CMP process in real time.
Various methods for detecting an end point have been introduced. One of the end point detection methods uses optical detection. A method using optical detection measures a remaining thickness or species of a material. In this method of end point detection, light having a specified wavelength range is projected onto and reflected from a wafer that is being polished. Then, the spectrum of the reflected light is analyzed to measure a remaining thickness or species of the material. Since the optical detection analyzes a surface of wafer being polished, the end point of the CMP process can be measured in real time independent of contamination of a slurry or an abrasion of an assembly. However, it is difficult to measure a thickness of multiple layers. Thus, a thickness measurement is not very accurate.
Another method of end point detection is a current detection method. In the current detection method, an end point is detected by measuring a current caused by a torque of a carrier that supports a wafer being polished. Similarly with the optical detection method, the current detection method can detect an end point in real time. The current detection has a disadvantage in that the torque and the current are irregular due to a density of patterns, a condition of the slurry, and an abrasion of the assembly.
In view of these disadvantages, the time control method is usually employed to control a CMP process. As mentioned above, in the time control method, the polishing process is performed for a predetermined process time, which has been previously set in accordance with a predetermined polishing rate based on a target material and a slurry. According to this method, an end point detector is not necessary, and the process is accurately controlled in the early state of the process because the polishing rate is maintained at the predetermined polishing rate. However, as an operation time of the polishing equipment progresses, the polishing rate begins to vary from the predetermined polishing rate.
FIG. 1 is a graph illustrating results of a CMP using a conventional method for controlling an end point of the CMP. In the graph, the abscissa indicates a process time and the ordinate indicates a polished amount.
Referring to FIG. 1, during the CMP process, a polishing pad, which polishes, i.e., abrades, the substrate, is conditioned by continuously supplying and exhausting a slurry, which contains byproducts, to and from the polishing pad while the substrate is being polished. Accordingly, the polished amount increases with respect to time, as shown in curve 1. Due to the time control method, the polishing is stopped at a predetermined polishing end time t1. The polishing end time is determined based on experimental data, which consists of polished amounts as compared to process times. The polishing end time can be selected at a time corresponding to a target value m1 of the polishing amount. As illustrated in FIG. 1, the chemical mechanical polishing continuously has a high polishing rate, i.e., the polished amount is steadily increasing, at the polishing end point because the polishing pad is being continuously conditioned.
However, an error with respect to the end time may occur during the CMP because the assembly becomes worn away and the polishing pad becomes glazed as an operation time of the polishing equipment elapses. More specifically, as the operation time of the polishing equipment elapses, the polishing rate decreases and only an amount m2 is polished by time t1. Thus, additional time t2 is necessary to polish the target value m1 of the target layer, as shown in curve 2. Since a variation of the polishing amount exists in the vicinity of the polishing end time, in the event that the polishing is stopped at the predetermined polishing end time t1, it is unavoidable that an error with respect to the polishing amount is high.